1. Field of the Invention
This invention relates to a curing system for use in adhesive compositions applicable to the hard tissues of the human body, such as teeth and bones. More particularly, the invention relates to a curing system for use in adhesive compositions wherein high adhesion to the hard tissues of the human body are required under wet conditions, said compositions being typically an adhesive for the treatment of a complex fracture of the bone, a filling agent for the fixation of artificial joints, a dental adhesive, a dental filling agent, and the like.
2. Description of the Prior Art
It has been common practice in the field of dental filling to cure a mixture of polymethyl methacrylate powders and methyl methacrylate monomer with a redox curing agent, i.e. a binary system consisting either of a peroxide and an amine or a peroxide and sulfinic acid (or a salt thereof) (BP No. 1,130,653). It is also known to cure a mixture of bisphenol-A diglycidyl methacrylate and triethylene glycol dimethacrylate in the presence of an inorganic filler using a binary redox curing system consisting either of a peroxide and an amine or a peroxide and sulfinic acid (U.S. Pat. No. 3,926,906). However, when a two-component redox curing agent of the peroxide-amine type is employed, the polymerizate exhibits substantially no adhesion to the hard tissues of human body, the bond strength under wet conditions being only on the order of 0 to 5 kg/cm.sup.2. Therefore, in dental practice, to make up for such deficiency in adhesion, mechanical-retaining means known as an undercut is provided in the tooth cavity so as to prevent displacement of the filling material after curing. This practice, however, has the disadvantage that the healthy tissue of the tooth is also removed. Because of the lack of bonding between the filling material and the tooth, this procedure provides only a poor marginal seal, which tends to cause a recurrence of the decay. The use of a two-component redox curing system consisting of a peroxide and sulfinic acid has been found not to be practical because of the extreme instability of sulfinic acid. Although this instability can be overcome by converting sulfinic acid to an amine salt thereof (BP No. 653,597), it has been found that such conversion to a salt causes a significant reduction in the rate of curing. Therefore, despite the knowledge that sulfinic acid and its salts can be components of curing systems for dental filling materials, to the best of our knowledge it has been, at best little used in practice. It is known that the degree of adhesion to the hard tissues of human bodies can be varied by using various components for the curing system. For example, the use of trialkylborane as a curing agent is known (Japanese Pat. Publ. No. 14318/1967). While this curing agent displays high adhesion to the dentin of the tooth, it has only poor adhesion to the enamel of the tooth, which contains little organic matter. Moreover, trialkylborane is not easy to handle for it is an unstable compound which, for instance, tends to ignite in air. It is also known that, in an organic peroxide-amine (or-sulfur compound) system, the addition of a hydroperoxide in a small amount is beneficial (U.S. Pat. No. 4,001,939). However, it does not possess adequate adhesion to the tooth and equivalent tissues. Thus, the current status of the art, no agent has been discovered that displays adequate adhesion to hard human tissues which is practical.
To obtain a satisfactory adhesion to the hard tissues of the human body, it is known to admix a monomer such as methyl methacrylate or bisphenol-A diglycidyl methacrylate with a small amount of a phosphoric acid ester monomer and cure the mixture (Journal Dental Research 35, p. 846, U.S. Pat. No. 3,882,600). According to the Journal Dental Research, dimethacryloxyglycerophosphoric acid is added to methyl methacrylate and the mixture is cured with a curing system consisting of benzoyl peroxide and p-toluenesulfinic acid. However, as discussed previously, p-toluenesulfinic acid is unstable and, moreover, the bond strength obtainable is low, 15 to 30 kg/cm.sup.2. U.S. Pat. No. 3,882,600 teaches a method which comprises adding glycerol dimethacrylate monofluorophosphate to a phthalate monomer and curing the mixture with a curing system consisting of benzoyl peroxide, N,N-dimethyl-3,5-dimethylaniline and dodecyl mercaptan. This curing system, however, tends to give off an odor which is objectionable in oral use and the monomer system employed has a physiologically injurious effect presumably due to the P-F bond.
A ternary curing system consisting of sulfinic acid, atmospheric oxygen (or dibenzoyl peroxide) and dimethylaniline is known to be an effective catalyst in the polymerization of methyl methacrylate or vinyl acetate (Bulletin Journal Chemical Society of Japan 32, 1079 (1959), Kobunshi Kagaku, Vol. 10, p. 441). However, this literature contains no reference to salts of sulfinic acid, nor does it even suggest the application of such a ternary system to an adhesive composition for the hard tissues of human bodies.
Among the properties required of such curing agents, there may be mentioned sufficient shelf life and an instant curing activity such that when used by a dentist or surgeon, it cures the monomer at room temperature without undue delay. It has found that, to meet the two requirements, the two components of a curing system, i.e. peroxide and amine (or sulfinic acid), are separately packaged and, when the system is used, the two packages are admixed (U.S. Pat. No. 3,926,906). However, this technique is not satisfactory when sulfinic acid is employed.
Other literature may also be cited as evidence that sulfinic acid (or its salt) is useful as a polymerization initiator. For example, German Pat. No. 1,138,226 and No. 1,123,824 describe the use of sulfinic acid in combination with a quaternary ammonium salt, or of a salt of sulfinic acid in conjunction with a quaternary ammonium salt. However, when put to use, these systems are by no means practical because of the low curing rates obtained.